The present invention relates to radiation spectroscopy generally and, more particularly, to novel method and apparatus using digital pulse de-randomization of signals produced by a radiation detector.
Radiation spectroscopy is performed by pulse height analysis of pulses from radiation detectors. The radiation interacts with the detector matter in a very short time. In semiconductor detectors, such interactions result in the creation of charge carriers that move from the point of interaction to the detector electrodes. The signal generation ends when all charges are collected. Therefore, the detector charge signal has a duration equal to the time interval from the moment of charge creation to the moment of full charge collection. If the effects of charge tapping and de-trapping can be neglected, then the duration of the detector current pulse is very short. The charge collection time depends on detector material, detector shape and the detector operating conditions. For most Ge and Si detectors, the charge collection time is in the range from a few nanoseconds up to a few hundreds of nanoseconds.
The current signal of the detector is superimposed on an equivalent background noise that is associated with the detector (leakage current) and the front-end electronics (preamplifier, bias circuit, etc.). It is, therefore, necessary to post-process (shape) the detector signal in order to improve (optimize) the signal to noise ratio. In modern spectroscopy systems, the duration and the form of the shaped pulses determine the pulse-processing time which is the minimum time required for correct estimation of the amplitude of the measured pulse. The duration of the shaped pulses can be 10 to 100 times longer than the response time of the detector. As a result, although the response of the detector is very fast, the pulse height measurement requires longer time due to pulse shaping that improves signal to noise ratio.
The event rate is the rate at which the radiation quanta interact with the detector and produce charge pulses. When radiation from radioactive sources is measured, the time intervals between adjacent events are randomly distributed. The distribution function is an exponential function that depends on the event rate. See Knoll, G. F., Radiation Detection and Measurement, 2nd ed., John Wiley and Sons, 1989. Therefore, for a given event rate, there is a finite probability that two or more events may occur during the pulse-processing time. This phenomenon is known as pulse pile-up.
The pulse pile-up affects both the pulse-height measurement and the throughput rate (recording rate) of the spectrometers. Ideally, only events that are free of pile-up should be accepted and recorded. For a paralyzable system (e.g. pulse shaper), the pile-up free rate is given by rexe2x88x922rT, were r is the event rate and T is the pulse processing time of the system. For instance, if the event rate is 50,000 events per second (cps) and the pulse processing time is 20 xcexcsec, the pile-up free rate will be xcx9c6,800 cps. If the events were periodic (equal time intervals between successive events), then for event rates less than Txe2x88x921, the pile-up free rate will be equal to the event rate.
One of the techniques to reduce the pile-up effects is pile-up rejection. See Knoll, supra. Devices, known as pile-up rejecters, are used in order to prevent recording of relatively large fraction of pile-up events. It is obvious that at high counting rates the rejection technique significantly reduces the throughput rate of the spectroscopy system. In order to improve the throughput rate, another technique has been employedxe2x80x94adaptive pulse processing. See Lakatos, T., xe2x80x9cAdaptive Digital Signal Processing for X-Ray Spectrometryxe2x80x9d, Nucl. Instr. And Meth., B47, pp. 307-310, 1990; and xe2x80x9cEDS Performance with Digital Pulse Processingxe2x80x9d, Mott, R. B. and J. J. Friel, in X-ray Spectrometry in Electron Beam Instruments, D. Williamns, J. Goldstein, and D. Newbury, eds., Plenum, N.Y., 127-157 (1995). This technique adjusts the pulse-processing time depending on the time interval between adjacent events. Although some improvement in the throughput rate can be achieved, the noise suppression (pulse shape) is not the same for all of the events causing undesirable peak distortion and broadening.
Accordingly, it is a principal object of the present invention to provide method and apparatus for eliminating or reducing pulse pile-up in radiation spectroscopy systems.
It is a further object of the invention to provide such method and apparatus that employ digital techniques.
It is another object of the invention to provide such method and apparatus that achieve de-randomization of the events with an average time interval between adjacent events equal to or longer than the pulse processing time.
It is an additional object of the invention to provide such method and apparatus that improve the throughput rate of systems with fixed pulse processing time.
Other objects of the invention, as well as particular features, elements, and advantages thereof, will be elucidated in, or be apparent from, the following description and the accompanying drawing figures.
The present invention achieves the above objects, among others, by providing, in a preferred embodiment, a method of digitally de-randomizing pulses in a radiation spectroscopy system, said method comprising the steps of: receiving an input signal representative of a radiation detector output; analyzing said input signal to derive separate event samples and background samples; storing said event samples and said background samples; and reading stored said event samples and said background samples and adjusting spacing in time between adjacent said event samples such that said event samples are spaced apart a time interval at least equal to pulse processing time of elements receiving an output of spaced apart said event samples and said background samples.